Television transmitter



Feb. 14,1939.

K. SCHLES INGER T ELEVISION TRANSMITTE R Filed May 16, 1936 Patented Feb. 14, 1939 TELEVISION TRANSMITTER Kurt Schlesinger, Berlin, Germany, assignor to .Radioaktiengesellschaft D. S. Loewe, Berlin- .Steglitz, Germany Application May 16,

1936, Serial No. 80,080

In. Germany May 18, 1935 .3 Claims.

The subject matter of the invention is a television transmitter, in which the synchronisation is performed in the known fashion by the fact that the carrier wave of the transmitter is set 55 :to zero during the synchronisation signals, whilst the image signals are incapable of attaining this condition. It is also assumed that the image :signals are impressed "in such fashion that max- 'im'um light intensity corresponds with a maxim mum aerial current, i. e., that the aerial cur-rent increases with the light intensity. It is already known to obtain synchronisation by means of a bright :strip provided on the dead angle of the film and scanned by a second photo-cell bring- ;l: ing the transmitter current to zero in the brief interval between the lines.

According to the invention the same effect is obtained by means of but 'one photo-cell and by using an amplitude filter.

In *Fig. 1a, 1 is a Nipkow-disc on which 2 is the projection of a positive film image. On one edge of this projection there is'provided -a very bright strip 3 the width of which is of about 5% of the horizontal width of the image. This edge strip 515 exceeds in brightness the white portions of the image and may be produced, for example, as reproduction. The contamination of the scanning disc by foreign matter is particularly disturbing. When using a positive film in the transmitter the same source of error in association with the scanning means represents a much smaller interference. In this case the black portions of the image are wholly devoid of interference, and merely the white portions of the image are reproduced with a somewhat smaller light intensity. Apart from avoidance of the error due to contamination there also exists otherwise a need for a television transmitter which is capable of operating with positive films and is not de- 40 pendent on negative films, and, on a phase reversing method.

The advantages of the optical synchronisation Y are to be maintained in the case of the solution according to the invention. The method de- 5 scribed by the applicant on a previous occasion,

in which auxiliary light is projected on to the total image area in even distribution, is precluded for the same reason as set forth relative to the transmission of negatives. It applies the 50 error consisting in the interfering reproduction of narrowed transmission image point holes to the television transmission of positive films.

In accordance, therefore, with the invention,

in agreement with Fig. 1a, there is projected on 55 to the Nipkow disc I of the transmitter the pro- 'jection of a positive film image 2, on which image there is located in the Width of approximately 5% of the horizontal width of the image a very bright edge strip 3 which exceeds in intensity the white portions-of the image. This edge strip F5 may be produced, for example, as reproduction of a gap by aspecial light source, or by the same light source as employed for the image reproduction. The transmitted light of the disc l, which is also shown from the side in Fig. 1b, impinges on aphoto-cell 25 connected to a photo-amplifier E5. The photo-amplifier '26 supplies at the out put a positive transmission, in which, therefore, positive potential values are supplied in the case of white portions of the image. The same, therel5 fore, in "the case of connecting the photo-anode with the grid of the first amplifier valve as shown, must have an uneven number, and in the case of cathodic connection of the photo-cell with the first grid an even number of amplifying valves. The modulating lead 4 leaving the amplifier controls the television transmitter 5 upwards in the manner described. Thepotential diagram at a is situated within a certain potential interval, corresponding to a completely opaque black up 25 to the brightest white of the image. By special means it is ensured that the black peak values of the image current at 4 always correspond to a prescribed and adjustable minimum potential value. 80

In Fig. 2 there is shown the time diagram corresponding to the potential occurring at 4. 6 is the value of said minimum potential which is adjusted once and for all and is maintained in. operation by means of a level-maintaining device, which does not permit a variation of this adjusted value. I is the potential reached only by the brightest points of the film image. According to the invention, the edge strip 3 in Fig.

1 is brighter in intensity than the brightest points 4,0 of the film. If its light intensity amounts to twice the maximum film light intensity, the corresponding potential is 8.

The following methods solve the problem according to the invention of converting all potential peaks which extend beyond the potential line 1 into black impulses, whereby even small amounts in excess of 'l are sufficient to reach or exceed the line 9the so-called ultra-black in the transmitter. To solve this problem the com- 60 bination is necessary of an amplitude filter and a phase reversing amplifier or a special currentdistributing grid fitted into the final stage of the transmitting amplifier 26. The amplitude filter is represented diagrammatically in Fig. 1b

by the apparatus HI. As soon as the image currents oscillating in the lead 4 exceed the value I (Fig. 2) the filter responds approximately at the threshold potential 1. There then appear at the lead I I, dependent on the circuit of the filter l0, positive or negative potential values corresponding with the excess beyond the threshold I. These potential values may be employed in various ways for bringing to zero the current of the transmitter 5. They may either be either be supplied to a special modulation grid of the modulation valve of 5, the eifect of which is greater than the efiect of the image modulation grid. Secondly, they may be conducted to an amplifier, for example l2, the output potential of which is greater to the extent of one order of magnitude than the potential supplied by 26 to the line 4. In this case the amplifier 4 shunted by a resistance l3, may be inserted in the line 4 so that when the threshold 1 has ben exceeded it supplies in opposition to the primary oscillation at 4 high negative impulses to the transmitter and accordingly brings to zero the latter. Finally, in accordance with the invention, the output of the amplifier I2 may be connected to a special grid in the amplifier 26 having a higher amplification factor than the normal Control grid excited by the preliminary stages of 26. There is common to all of the methods referred to the use of an amplitude measuring device It and use of the excess potentials in white transmitted thereby for obtaining the effect of reducing the aerial current of the transmitter to zero.

For the sake of better comprehension there is illustrated in Fig. 3 a circuit for performing the method of the modulation with two modulation grids.

In Fig. 3, I4 is the final valve of the image amplifier 26. The same acts on the anode resistance I5. By way of the lead 4 it is connected with the modulating valve I6 of the transmitter. There is preferably employed a circuit, wherein the lead 4 conducting the modulation potential is connected to the screening grid I! of the valve is, whilst the anode circuit of this valve is tuned to a harmonic of the grid circuit of the same valve and this circuit is excited with constant potential from a control transmitter l8. The valve IS, in accordance with the invention, requires to have a second modulating grid, for which there may be employed the intercepting grid [9 or, even better an additional grid 20. The grid I9 is connected to a positive bias 2|, which is of such amount that the screening grid I1 is just currentless. The grid 20, which will be referred to as distributing grid, can then again have the potential of the cathode, and is, therefore, already currentless in itself. For amplitude filtering purposes there is employed a valve 21 which is operating as anode bend rectifier. The grid thereof is negatively biassed to such extent by a battery 22 that only the over-white positive peaks of the lead 4 are able to release the valve 21 by way of a coupling condenser 23. In the anode circuit there then occur negative impulses, which are passed through to the distributing grid 26 by the coupling condenser 24. According to the invention, the steepness of modulation of the control system 21/20 is made to be greater than the corresponding steepness of the system 4/ l1. There then disappears completely the aerial current in each case, even if the grid I1 is positive. This object may be readily accomplished by means of small-mesh grids 20 and by good amplification in the anode bend rectifier 21.

I claim:

1. In a television transmitter comprising an only scanning element used for scanning the image and used for producing the synchronizing impulses by scanning a white margin on said image, the light intensity of which is higher than that of any point of the image, a photo current amplifier controlled by the current of said scanning element, a filter in the output of said photocurrent amplifier having a threshold value so as to be passed by said impulses only, a reversing stage being connected to the output of said filter, the output of said reversing stage being connected to the input of the high frequency generator of the transmitter so as to bring the transmitted energy to zero by said impulses.

2. In a device according to claim 1, said reversing stage being an amplifier amplifying the synchronizing impulses oppositely directed to the image signals.

3. In a device according to claim 1, a highfrequency generator provided with a modulation tube having two grids in series, one grid being connected to the output of said photo-current amplifier, the other grid to the output of said special amplifier, the controlling effect of the latter grid being suiiicient to block said modulating tube.

KURT SCHLESINGER. 

